Microsoft Windows and Apple Macintosh
icons are very similar. They both have several image formats embedded
in the same icon. But the file format is totally different. You cannot
use Macintosh icons under Windows because the file format is not compatible.
You must convert them to Windows ICO file format (with IconWorkshop of
course!).

All kinds of formats can be included in
a Windows ICO file (you can even create non-square icons). With Macintosh
icon files this is not possible. Only some strictly-defined image formats
can be included.

Axialis IconWorkshop supports the following
Macintosh icon files: ICNS, RSC and BIN.
Icons from system 7.5, 8.5, 9.x and 10.X are supported. You can easily
open them into the Icon Editor and save them as Windows ICO format. During
this operation, we recommend you to remove special Macintosh formats
which are useless under Windows.

The new OS-X icons can contain very large
image formats (128x128, 256x256 - 32 bits). Those formats are also supported.
Although those image sizes are not actually used in Windows, it's a good
idea to start from hi-res sizes to create a new Windows icon.

Here
is a summary of image formats you can find in Macintosh and Windows Icons:

This is the simplest way to proceed. All the Windows image formats may not be copied because the Macintosh icon file format is not open (only the Macintosh formats as specified above are supported). For example, the 24x24 image format is not supported in Macintosh icons.

1.
Open the Windows icon. Choose File/Open...
The icon is opened in a document window.

This operation is not a conversion, it's a creation of a new icon. It will not preserve all the existing formats. It will create a new icon based on the largest and most coloured image included in the original Windows icon (usually 256x256, 128x128 or 48x48 with alpha channel).

1.
Open the Windows icon. Choose File/Open...

2.
When the icon is opened, select the largest and the "most coloured"
image format in the list (best with 256x256, 128x128 or 48x48 with alpha channel).

3.
Right-click on the selected image format in the list (),
a menu opens. Select "New
Macintosh Icon Project from this Image..." ().

4.
A large dialog box opens. In the Name
group (), enter the icon project name. Do not specify any
extension.

5. In the Image Formats group, select
the image formats you whish to add in the icon ().

6. You can also add other formats (256x256 to be used in Tiger). When done, click OK. The new Icon Project document window is created with all formats embedded.

7. Save your
icon to Macintosh format. choose File/Save
and choose a new name (.ICO) for this icon.